1. Field of the Invention
The invention relates generally to bismuth-containing oxide films. In particular embodiments, the present invention concerns novel processes for manufacturing bismuth-containing oxide thin films by atomic layer deposition.
2. Description of the Related Art
Bismuth is a component of several technologically important binary and multicomponent oxide thin film materials, particularly the ferroelectric oxides Bi4Ti3O12, (Bi,La)4Ti3O12, BiFeO3 and SrBi2Ta2O9, and the superconducting oxide Bi2Sr2CaCu2O8+x. The ferroelectric bismuth oxide films have great potential for use in ferroelectric memories. Such memory devices are likely to be provided with capacitors in the form of 3-D structures, which means that good conformality of the ferroelectric film will be important for proper function of the memory.
Atomic layer deposition (“ALD”) refers generally to vapour deposition-type methods in which a material, typically a thin film, is deposited on a substrate from vapour phase reactants. ALD is based on sequential self-saturating surface reactions. ALD is described in detail in U.S. Pat. Nos. 4,058,430 and 5,711,811, incorporated herein by reference.
According to the principles of the ALD, the source chemicals (also known as “reactants” or “precursors”) are separated from each other, such as by inert gas, to prevent gas-phase reactions and to enable the above-mentioned self-saturating surface reactions. Surplus source chemicals and reaction by-products, if any, are removed from the reaction chamber before the next source chemical is introduced into the chamber. Undesired gaseous molecules can be effectively expelled from the reaction chamber, for example, by keeping the gas flow speeds high with the help of an inert purging gas. The purging gas can be used to push the extra molecules towards a vacuum pump, which is also used for maintaining a suitable pressure in the reaction chamber. ALD provides controlled film growth as well as outstanding conformality.
Based on its general properties, ALD is a potentially attractive alternative for deposition of bismuth-containing oxide thin films. The main problem in the art has been finding appropriate bismuth—oxygen source chemical combinations for depositing bismuth oxide.